1. Field of the Invention
The present invention relates to an automatic focusing apparatus, and more particularly, although not exclusively, to an automatic focusing apparatus for an optical system that irradiates a surface to be processed or machined with a light flux of a light source.
2. Description of the Related Art
Conventionally, in processing a workpiece by irradiating its surface with a laser beam, it is occasionally required to confirm the position of the irradiating point of the laser beam or required to observe the processed condition of the workpiece. Furthermore, condensing the laser beam for processing the workpiece may be required. When such a processing operation is carried out, a focus adjustment relative to a surface which is irradiated with the laser beam can be performed for an optical system used for observation or for an optical system used for laser processing. Furthermore, to improve the efficiency of the laser processing operation, an automatic focusing apparatus for automatically adjusting focus is provided.
The automatic focusing method can be employed to directly measure the distance between the optical system (i.e. an objective lens) and the laser beam receiving surface. In such a case, in view of improving the accuracy in the automatic focusing adjustment, measurement of the distance between the optical system (i.e., an objective lens) and the laser beam receiving surface may be done at the position close to the portion where the processing operation using the laser beam is performed. Furthermore, in recent years, the processing method uses a laser beam to cut a substrate by condensing the laser beam inside the substrate at a plurality of regions of the substrate surface.
As one example, a silicon wafer or other semiconductor substrate is required to be accurately cut into chip pieces. Conventionally, a substrate is cut by rotating, at a high speed, a circular blade having a width of several tens μm to several hundreds μm so that the abrasive material on the blade surface can grind the substrate. This method is generally referred to as a blade dicing method.
According to this method, to reduce heat generation or abrasion during the cutting operation, it is usual to spray cooling water on the cut surface. In such a case, parts of the substrate or fine particles of abrasive material generated during the cutting operation can be mixed in the cooling water with other waste such as adhesive particles of an adhesive tape used to fix the substrate to a processing table and can spread in a wide region.
Especially, in a case where the substrate is a semiconductor substrate, numerous fine functional elements are formed on the substrate surface. Reliability of these functional elements can be greatly lessened.
To address these issues, one can perform the cutting operation in a dry environment without using the cooling water. To this end, to cut the substrate, a processing method using a laser beam having a wavelength highly absorbable by the substrate can be employed to condense the laser beam on the substrate surface.
However, according to this method, not only the portion directly irradiated with the laser beam but also its peripheral portion on the substrate surface can melt. The logic circuits on the semiconductor substrate can be damaged. Furthermore, the process of laser processing operation progresses from an incident side of the laser beam to an exiting side so that melting of the substrate successively occurs. Accordingly, fine particles in a molten state will soon and successively harden and remain as dust particles on the substrate surface. In this respect, like the above-described blade dicing operation, the laser processing operation is not free from the problem of dusts.
To address such problems, Japanese Patent Application Laid-open No. 2002-192370 and Japanese Patent Application Laid-open No. 2002-205180 discuss a processing method that uses a laser beam highly absorbable by a substrate so that the substrate is cut by utilizing internal beam-condensing of the laser beam.
More specifically, a laser beam having a specific wavelength that shows high permeability relative to a substrate, i.e., a material to be processed, is used to condense the laser beam inside the substrate, thereby setting a start point of the cutting operation in a predetermined internal region of the substrate. This processing method can realize an excellent cutting operation accompanied with less waste because no melt region is formed on the substrate surface. According to an apparatus for processing or cutting a workpiece or a substrate by utilizing the irradiation of such a processing laser beam, the laser beam is condensed inside the substrate via a microscope objective lens that is used for observing a processing position. The laser processing operation is performed by driving the substrate with a stage or comparable actuator along a predetermined processing line or a predetermined cutting line while keeping the internal beam-condensing condition of the laser beam.
To perform the above-described operation using a laser beam for processing a workpiece or cutting a substrate, one can continuously observe the condition of the laser beam irradiating the substrate surface with an objective lens during a processing or cutting operation of the surface to be irradiated (i.e., a laser beam receiving region which is irradiated with the laser beam).
However, the objective lens used in this case is also used for irradiating the laser beam receiving region with the laser beam or for condensing the laser beam. In other words, it can become necessary to simultaneously satisfy two conditions, i.e., a condition for observing the substrate surface being processed and a condition for controlling the irradiation or condensing of the laser beam.
However, according to the above-described automatic focusing apparatus or the laser processing apparatus, the material of the surface to be processed tends to soften, melt, or deform during a laser beam processing operation due to irradiation of the laser beam, at the center of the optical axis of an irradiation optical system that provides a processing laser beam (i.e., at the processed surface irradiated with the laser beam).
Therefore, if the position including the center of such an optical axis is regarded as a reference position for the distance measurement performed by the automatic focusing apparatus, it is difficult to accurately measure the distance between the optical system (i.e., an objective lens) and the laser beam receiving surface.
Moreover, the above-described laser cutting apparatus can form an internal processing region inside the substrate, and accordingly there is no intent to directly process the substrate surface. However, it is the fact that the region including the center of the optical axis of the processing laser beam is irradiated with the processing laser beam when this laser beam is condensed inside the substrate. Thus, the substrate surface corresponding to the above region is somewhat processed. This is one reason why one may not want to designate the central portion of the optical axis as a substrate-side reference position for the distance measurement performed by an automatic focusing apparatus.